The present disclosure relates to a wireless communication system. More specifically, the present disclosure relates to: a method for transmitting a physical random access channel (PRACH) in at least one carrier from among a first carrier group on the basis of a channel sensing result, receiving a random access response (RAR) in at least one carrier from among a second carrier group in response to the transmission of the PRACH, and transmitting a physical uplink shared channel (PUSCH) on the basis of the RAR; and an apparatus therefor.